Quote
The shifted byte is promoted to an int.
Why?
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They be the rules, and the compiler knows the rules. So do you 
ISO International Standard ISO/IEC 14882:2014(E) – Programming Language C++
http://en.cppreference.com/w/cpp/language/operator_arithmetic#Conversions
Bitwise shift operators
The bitwise shift operator expressions have the form
lhs << rhs (1)
lhs >> rhs (2)
- left shift of lhs by rhs bits
- right shift of lhs by rhs bits
For the built-in operators, lhs and rhs must both have integral or unscoped enumeration type. Integral promotions are performed on both operands.
The return type is the type of the left operand after integral promotions.